In general, water repellency refers to a water-resistant property or hardness to get wet by water. The super water repellent surface technique is a kind of surface modification technique to control the wetting of a surface. Through physical or chemical surface modification of a solid surface, the technique aims at maintaining the contact angle of the solid surface with water at 150° or more.
A representative model of a super water repellent surface is the leaves of lotus, the surface of which has numerous micro-sized cilia and is coated with a waxy substance.
The wax coating also has been identified to have a regular nanostructure. It is known that such a micro/nano hierarchical structure results in a super water repellent surface and leads to a self-cleaning effect.
Recently, researches about preparing a super water repellent surface having more improved water repellency by mimicking the micro/nano hierarchical structures existing in the nature are drawing attentions.
The researches on super water repellency are gaining attention not only in the field of science but also in various industries, with regard to construction materials, cosmetics, fiber treatment, electric/electronic components, etc.
In general, the surface of glasses used in vehicles and buildings have a small contact angle with water of about 20-40° Thus, when it rains, water flows on the glass surface while forming an inhomogeneous water film.
Such an inhomogeneous water film leads to light scattering on the windowpane of vehicles, thereby interfering the drivers vision especially at night or during raining. It also leads to an easy contamination of the glass surface of building windows with dust, yellow sand, and so forth.
By significantly reduce the surface energy of the glass surface, it will be possible to make the shape of water droplets spherical, so that they may roll down off the surface, and the glass will hardly get wet by water.
Such a glass is called the super water repellent glass. When used for vehicles, the super water repellent glass can prevent the distortion of vision caused by the inhomogeneous water film and offer a clear vision, thereby preventing car accidents.
And, if the super water repellent glass is used in the windows of large high-rise buildings, which are difficult to clean, impurities attached on the super water repellent glass are easily removed as spherical water droplets roll down off the glass surface because of low surface energy. Such a self-cleaning effect is very advantageous in the maintenance of buildings.
The contact angle of a water droplet on a solid surface is the index of the water repellency. In general, a surface having a contact angle of 90 or more is called a water repellent (hydrophobic) surface. And, a surface having a contact angle of 110 to 150 is called a highly water repellent surface. If the contact angle is 150 or more, the surface is called a super water repellent surface.
Formerly, since the 1950s, the researches on the development of the water repellency technology were focused on developing materials having a chemical structure leading to a low surface energy. In the 1980s, it was discovered that not only the surface chemical property but also the geometric spatial structure is an important factor affecting the super water repellency.
Water repellent films coated with various fluorine based materials are an example of utilizing materials with low surface energy. But, with the low surface energy alone, it is difficult to obtain a contact angle of 150 or larger. Hence, the control of the surface microstructure is required to obtain a super water repellent material having contact angle of 150 or larger.
It is reported that a surface having a water contact angle of 132 to 170 was prepared through nanosphere lithography and dry etching using oxygen plasma (Peilin Chen, et al. Chem. Mater., vol. 16, no. 4, 561, 2004).
However, this research is no more than assembling polystyrene nanospheres on a gold film in a single layer or double layers and modifying the size and form of the polystyrene nanospheres using oxygen plasma. It offers unsatisfactory water repellency because the solid surface itself is not etched to form a concavo-convex structure.
According to Korean Patent Publication No. 1997-007696, “Preparation of water repellent glass and product prepared thereof”, a solution containing a silicon alkoxide based compound and an organic solvent is hydrolyzed to form a silica film and a water repellent layer is coated to prepare a water repellent glass. But, the water contact angle is only around 100° and the super water repellency is not attained.
Korean Patent Publication No. 1999-0001695, “Concavo-convex silica film having superior durability and water repellent glass using the same” provides a water repellent glass prepared by coating a water repellent on the surface of a concavo-convex silica film. However, because the colloidal silica resulting from the hydrolysis and polucon-densation of a silane compound is coated on the surface to obtain a concavo-convex structure, it is impossible to obtain a regular concavo-convex structure with wanted scale.